In Book II chapter viii of the Essay Locke provides two related criteria by which primary qualities are to be distinguished from secondary qualities: first, primary qualities are inseparable from the bodies in which they inhere, whereas secondary qualities are not. Second, secondary qualities are merely the observable effects of primary qualities and not real in the same sense in which primary qualities are, indicating an ontological dependence of the former on the latter. Locke uses these two criteria of demarcation to argue to a conclusion about our ideas of primary and secondary qualities, namely that ideas of primary qualities resemble those qualities but our ideas of secondary qualities do not. I will argue here that Locke’s two criteria are flawed and that he is inconsistent in applying them to qualities, and that his mistakes in correctly drawing the ontological distinctions between primary and secondary qualities result in his misapplication of those distinctions to the epistemological realm.
First, in II.viii.9 Locke argues that primary qualities are “utterly inseparable” (134 in the Nidditch edition) from the body which possesses them. No matter what permutations a body goes through, it must possess the same set of primary qualities. Sense tells us that primary qualities persist through change in every bit of matter which has “bulk enough to be perceived” (134) by the senses, and the mind reasons that these properties must persist in particles of matter too small to be perceived. Secondary qualities, on the other hand, are those which are altered by changing the arrangement of matter in the body. Locke gives as examples of primary qualities solidity, extension, figure, and mobility (135); secondary qualities are things like color, taste, and odor.
There are problems with the line that Locke has drawn between primary and secondary, even using his own rationale. Solidity and extension here make sense as primary qualities using Locke’s criterion of inseparability, but it is difficult to see how figure, as a category of properties, rather than a property itself could be regarded as primary. An object possesses a shape, (it is triangular or circular, for example), but figure itself is not really a property of a particular parcel of matter; or, conversely, if Locke claims that the fact that no matter what you do to matter it still has some shape, this makes figure a primary quality, Locke owes an argument to tell us why categories of secondary qualities such as color, taste, and smell would not qualify as primary as well. It can’t be that some bits of matter are too small for us to taste, for example, and thus have no taste; Locke allows that the mind must use reason to conclude that solidity and extension remain qualities of bodies too small to be directly perceived. Locke gives us no reason not to conclude that minute parcels of matter would also possess some color, taste, or odor by the same line of thought. Mobility also seems out of place as a primary quality; mobility is a description of what can be done to a parcel of matter, not a description of some property inhering in it. Including mobility as a primary quality does not seem to be any more warranted than including “unable to be sent to the moon by mediaeval catapult” as a primary quality inseparable from any parcel of matter.
Locke uses an almond as an example to show the difference between primary qualities, which are in an object, and secondary qualities which are not. If an almond is pounded, it goes from being sweet and white to brownish and bitter, which Locke argues indicates color and taste are not in the almond properly (139). Merely by moving the parts of an almond around, the qualities it seems to possess to our senses of sight and taste are changed dramatically. What Locke fails to notice, however, is that the almond also goes from being almond shaped and hard to amorphous and pasty, but for some reason this does not indicate to Locke that neither its figure nor texture is truly in the almond. His inclusion of categories of properties like figure and texture in the class of primary qualities and exclusion of categories of properties like color and taste weaken his attempt to draw the distinction, since he does not give us a principled reason for including some genera of qualities but not others. The almond before and after has a shape, a texture, a color, and a flavor, each of which is altered by pounding it.
Locke’s second way of explaining the difference between primary and secondary qualities is to argue that since secondary qualities are “nothing in the Objects themselves” (II.viii.10, 135) they are merely the capacities of objects to produce particular ideas in perceivers. Whereas primary qualities really inhere in objects, secondary qualities are the product of the interaction between objects and the perception of objects, produced in perceivers by the “Bulk, Figure, Texture, and Motion” of the insensible parts of matter; our ideas of secondary qualities are the effects of the capacities of objects. These have an ontological status similar to other properties of objects or phenomena; a fire’s ability to produce warmth in us is the same type of phenomenon as its ability to melt a chunk of wax; in neither case (the ability to warm or the ability to melt wax) is the effect a discrete quality in the fire, but is instead an effect of the fire’s powers, which are direct consequences of the primary qualities of fire. Secondary qualities of bodies are thus the products of the causal powers the primary qualities of bodies.
Locke uses the example of a piece of porphyry (II.viii.19, 139) to show that changes in the appearance of the color of porphyry under various conditions indicate that the color is an effect of the primary properties which remain unchanged. Porphyry, when unobserved or when no light shines on it, has no color; thus, Locke argues, there is no color truly in the porphyry, since the presence or absence of light has no real effect on the particles of matter which comprise the chunk of stone. If porphyry truly possessed color as a primary quality, it would appear to be the same color in all circumstances. This conclusion shows that Locke conflates presence or absence of a property with the presence or absence of suitable conditions for observing the property, since our inability to observe a property does not entail its absence. This point is one that Locke himself recognizes in his argument that we can conclude, even if we can’t perceive, that minute parcels of matter retain their solidity and extension. Ignoring his own line of reasoning regarding solidity, Locke confuses an epistemic claim about the availability of a quality for observation with an ontological one.
This confusion of epistemic and ontological status is reflected in the conclusion about our ideas about qualities that Locke draws based on his discussion about the qualities themselves. Locke concludes from his discussion about the nature of primary and secondary qualities that ideas of primary qualities “are Resemblances” (137) of the qualities. Locke does not give the argument that leads from his first distinction (primary qualities are inseparable, secondary are not) to his conclusion that our ideas of the first resemble them, but the ideas of the second do not. One way to reconstruct the line of reasoning, however is to view the conflation of epistemic and ontological properties as at work here, as well. Locke assumes that the sensitivity of our perception of secondary qualities to observation conditions picks out a real state of affairs in the world; since the color of porphyry changes in different light, then our idea of the color of porphyry must not resemble the real properties of porphyry which produce the color. Conversely, since the primary qualities of bodies persist through changes or observation conditions, we can assume that our ideas of those qualities do resemble the qualities.
The problem, however, is that even if we were to accept at face value Locke’s conclusion that the fact of the variability of the appearance of secondary qualities means that our ideas of those qualities cannot resemble the qualities, this is still not sufficient reason to conclude that our ideas of primary qualities do resemble those qualities. The conclusion simply does not follow from the premise. Locke could add a premise, something like “Ideas resemble the qualities to which they refer if and only if the quality referred to is in the body producing the idea”, but this is essentially what he is trying to conclude. In addition, if Locke’s distinction between primary and secondary qualities based on our ability to perceive them in different conditions is flawed, as I have claimed, then his account leaves us no reason for expecting there to be a sharp difference in our ideas of qualities based on an unspecified “resemblance” relation. This problem is compounded by the fact that Locke recognizes that our ideas of primary and secondary qualities are produced in the mind by the same set of causal mechanisms (II.viii.11-14). If the boundary between primary and secondary qualities does not operate the way Locke says it does, and if our ideas of both types of qualities and produced in the same way, there is little reason for us to believe that those ideas will show distinctly different relations to the qualities to which they refer.
In sum, Locke’s distinctions between primary and secondary qualities are problematic. The former category includes particular qualities as well as classes of properties, whereas classes of properties in the latter category are not taken into consideration as either primary or secondary. In addition, in reference to secondary qualities, Locke fails to distinguish between the existence of properties and the observation conditions which make our perceptions of those properties possible. Locke invokes this distinction in reference to his primary qualities, but does not allow the same line of thought to apply to secondary qualities. Using the poorly drawn boundary between primary and secondary qualities and extending the conflation of existence of properties with their perceptibility leads Locke to the unjustified conclusion that our ideas of primary qualities stand in a wholly different relation to the world than our ideas of secondary qualities.